Facile electrosynthesis of π-extended porphyrins

Article abstract of DOI:10.1039/c4cc02759k

The facile electrosynthesis of π-extended porphyrins is demonstrated for a series of Zn(ii), In(iii), Ir(iii) and free-base meso-substituted derivatives containing the 4,7-dimethoxynaphthalen-1-yl substituent. Electrochemical data suggest that the overall process initially involves two stepwise one-electron oxidations, followed by an intramolecular oxidative aromatic coupling to give the electrooxidized π-extended porphyrin. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc02759k

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Facile electrosynthesis of p-extended porphyrins†
Yuanyuan Fang,a Dominik Koszelewski,b Karl M. Kadish*a and Daniel T. Gryko*b
Cite this: Chem. Commun., 2014,
50, 8864
Received 14th April 2014,
Accepted 14th June 2014
DOI: 10.1039/c4cc02759k
www.rsc.org/chemcomm
The facile electrosynthesis of p-extended porphyrins is demonstrated was first applied to the synthesis of porphyrinoids by Sessler and
for a series of Zn(II), In(III), Ir(III) and free-base meso-substituted Bucher.18 At the same time electrochemistry is a unique tool to
derivatives containing the 4,7-dimethoxynaphthalen-1-yl substituent. elucidate the order of steps in oxidative aromatic coupling. We
Electrochemical data suggest that the overall process initially involves envisioned that such electrochemical reaction described in our
two stepwise one-electron oxidations, followed by an intramolecular initial study for fusion of a nickel porphyrin19 would also occur
oxidative aromatic coupling to give the electrooxidized p-extended for porphyrins containing a variety of metal ions as well as for
porphyrin.
the free-base porphyrins where chemically induced fusion reac-
tions are yet to be demonstrated. As a test of this hypothesis,
Both p-expanded and p-extended porphyrins have been widely we set ourselves the goal to perform the electrosynthesis of
used as building blocks for supramolecular and organic materials p-extended porphyrins for four types of compounds: (a) an easily
chemistry.1–8 The intrinsically high electron-density of porphyrins oxidizable formally four coordinate zinc(II) porphyrin, which we
is responsible for the fact that oxidative aromatic coupling9 has wished to directly electrosynthesize as a p-extended derivative
recently become the most popular method for p-extension of without concomitant removal of the Zn(II) central metal ion;
these compounds.2,10 Although numerous aromatic hydrocarbons (b) a five coordinate indium(III)-porphyrin, which possesses a
and heterocycles have been fused with porphyrins over the last relatively high oxidation potential; (c) a six coordinate Ir(III)
20 years, many challenges still remain, the most important of porphyrin which also possesses a high oxidation potential and
which are an unpredictability of the reaction outcome, contami- (d) a free-base porphyrin, the latter of which has proven all but it
nation from mono-chlorinated by-products (FeCl3)11 and the lack is impossible to be synthesized in its p-extended form from
of a successful fusion reaction with free-base porphyrin deriva- ‘regular’ free-base porphyrins using chemical methods.
tives. As a matter of fact, despite the publication of hundreds of
manuscripts on the topic, only Zn-, Cu- and Ni-complexes of to be successful in chemical synthesis of the same p-extended
porphyrins have been successfully oxidatively coupled.2,10–15
porphyrins i.e. porphyrins possessing three mesityl groups and one
We resolved to use the same basic architecture as was shown
Further progress in this methodology depends on cracking 4,7-dimethoxynaphthalen-1-yl unit.11 The rationale behind this
down the mechanism of this reaction. Various mechanisms choice was the possibility to compare properties of the chemically
have been proven to operate in oxidative aromatic coupling fused products to that of the products generated by an electro-
such as coupling of aryloxy radicals, radical substitution, and chemically induced oxidation process. In the chemical fusion
cation-radical dimerization.9 Electrochemically driven oxidative method, mixed-condensation afforded the porphyrin Open-H2,
aromatic coupling is a useful method which can afford products which was subsequently transformed into the zinc complex
on the electrode surface without contamination16 as recently Open-Zn, which was then chemically oxidized with Fe(ClO4)3ÁH2O
demonstrated by Waldvogel and co-workers.17 This approach to afford Fused-H2 in 52% yield.11 As described previously, the
Lewis acid character of Fe3+ causes quantitative removal of zinc
a Department of Chemistry, University of Houston, 4800 Calhoun Road,
during oxidative aromatic coupling. The porphyrin Fused-H2 was
subsequently metallated following known procedures to afford the
In(III), Zn(II) and Ir(III) complexes (Scheme 1).
Measured half-wave or peak potentials for reduction and oxida-
tion of the tetra-mesitylporphyrin (TMP) and fused porphyrins
in PhCN are summarized in Table 1 while the redox potentials
in CH2Cl2 are given in Table S1 (ESI†). These tables also include
77204-5003, USA. E-mail: kkadish@uh.edu
b Institute of Organic Chemistry, Polish Academy of Sciences, Kasprzaka 44/52,
01-224 Warsaw, Poland. E-mail: dtgryko@icho.edu.pl
† Electronic supplementary information (ESI) available: Full synthetic and analytical
data of compounds TMP-Zn, TMP-InCl, TMP-Ir(CO)Cl, Open-Ir(CO)Cl, Open-InCl,
Fused-H2, Fused-Zn, Fused-InCl, Fused-Ir(CO)Cl as well as copies of 1H NMR. See
DOI: 10.1039/c4cc02759k
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oxidized in two one-electron steps located at E1/2 = 0.53 and
0.85 V in PhCN, 0.1 M TBAP. This is compared to the Open-Zn,
which exhibits two reversible one-electron oxidations at E1/2
=
0.81 and 1.12 V under the same solution conditions. The
product of the one-electron oxidation of the Open-Zn is stable
on the cyclic voltammetry time scale and it is also stable after
holding the potential for 2 min at 1.05 V and then reversing
the potential sweep direction to reduce the Open porphyrin
p-radical cation to its neutral form again. This is shown by the
top cyclic voltammogram in Fig. S1a (ESI†).
The second oxidation of Open-Zn also occurs on the porphyrin
core followed by electron-transfer leading to
a bis-radical
(Scheme 2). The doubly oxidized Open-Zn generated in PhCN at
E1/2 = 1.12 V is unstable on the cyclic voltammetry timescale of
0.10 V sÀ1 and undergoes a slow chemical conversion to the dication
of the Fused-Zn, as demonstrated by the decreased current for the
second oxidation on the return sweep and the appearance of a new
cathodic peak at Ep = 0.48 V (see Fig. S1a, ESI†).
Scheme 1 Chemical and electrochemical synthesis of p-extended
porphyrins.
Additional evidence for electrosynthesis of the p-extended
porphyrin is provided by the cyclic voltammogram in Fig. S1b
(ESI†) where the potential was held for 2 min at 1.50 V before
initiating the scan in a negative direction. Under these condi-
tions, a larger amount of the Open-Zn is consumed and the
redox process associated with the electrosynthesized Fused-Zn
at E1/2 = 0.51 V exhibits an increased peak current, consistent
with larger amounts of this material being formed at the
electrode surface. A comparison of the cyclic voltammogram in
Fig. S1b (ESI†) with that of the chemically synthesized Fused-Zn
in Fig. S1c (ESI†) clearly shows that the same chemical species
has been formed by electrosynthesis in PhCN as that by classical
chemical synthetic methods.
Table 1 Half-wave or peak potentials (V vs. SCE) in PhCN, containing
0.1 M TBAP. Scan rate of 0.1 V sÀ1
Oxidation
Reduction
M
Ring
2nd
1st
1st
2nd
H–L gap (V)
H2
TMP
1.41
1.24b
1.01
1.18
1.12
0.85
1.14
1.14
1.00
1.57
1.40b
1.13
1.64
1.43b
0.85
1.12
1.10
0.71
0.82
0.81
0.53
1.14
1.14
0.74
1.24
1.21
0.85
1.27
1.22
0.53
À1.19
À1.19
À1.08
À1.38
À1.36
À1.25
À1.30
À1.24
À1.20
À1.05
À1.01
À0.97
À1.17
À1.15
À1.23
À1.69
À1.68
À1.56
—
À1.85
À1.71
—
À1.84
À1.80b
À1.58
À1.55
À1.46
À1.39
À1.74
À1.69
2.31
2.29
1.79
2.20
2.17
1.78
2.44
2.38
1.94
2.29
2.22
1.82
2.44
2.37
1.76
Open
Fused
TMP
Open
Fused
TMP
Open
Fused
TMP
Open
Fused
TMP
Zn
Nia
InCl
Ir(CO)Cl
Open
Fused
a
b
Data taken from ref. 19. Peak potential at a scan rate of 0.1 V sÀ1
.
data for the related starting compounds with a single meso-
naphthalene group.
The similarity in the first reduction and first oxidation
potentials of compounds having the TMP and Open macrocycles
is related to the similar substituent effects of the trimesityl and
naphthalene groups and to the similar sites of electron transfer,
which in this case is the conjugated p-ring system of the
porphyrin. Similar potentials are not observed for the second
oxidation of the TMP and Open macrocycle compounds where
much larger differences are observed suggesting different sites
of electron transfer for this reaction in the two series of com-
pounds. The DE1/2 between the TMP and Open porphyrin
derivatives ranges from 170–210 mV in the case of the free-
base, InCl and Ir(CO)Cl compounds to 60 mV in the case of the
Zn(II) complexes.
As shown in Table 1, the chemically synthesized Zn(II)
porphyrin with a p-extended system Fused-Zn is reversibly
Scheme 2 Mechanism of electrochemical oxidative aromatic coupling.
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In summary, electrochemical evidence suggests that the
Communication
mechanism for conversion of the Open-Zn to the Fused-Zn
involves two one-electron oxidations followed by a radical
coupling to give the Fused-Zn dication as shown in Scheme 2.
This doubly oxidized porphyrin with an extended p-system is
then reduced by two one-electron transfers at 0.85 and 0.51 V to
give the final, neutral Fused-Zn. Both the first and second one-
electron abstractions of the Open-Zn occur at the porphyrin
core, which is governed by the order of electrochemical oxida-
tion potentials (Table 1).
Scheme 3 Detailed mechanism of electrochemical oxidative aromatic
coupling for free-base porphyrins.
The spectroelectrochemistry (Fig. 1) gives evidence that the
slightly different mechanism occurs in the case of the InCl
derivatives where the Open-InCl porphyrin is converted to the
doubly oxidized Fused-InCl porphyrin after controlled potential
oxidation, followed by controlled potential reduction of the
electrogenerated product. In this case however, after first
oxidation the second one occurs on the naphthalene moiety
leading directly to a bis-radical (Scheme 2).
A similar mechanism also occurs for conversion of Open-
Ir(CO)Cl to the Fused Ir(III) derivatives but slight differences are
seen in the UV-vis spectra of the chemically and electrochemically
generated p-extended porphyrins. The chemically synthesized
Fused-Ir(CO)Cl has a split Soret band at 463 and 493 nm and a
Q band at 686 nm (see ESI† Fig. S4 and S6). Also in this case
second one-electron abstractions occur at the linked naphthalene
group which is electroactive and oxidized at 1.12 V in PhCN,
0.1 M TBAP.19 This difference is related to the fact that the
oxidation potential of the dimethoxynaphthalenyl unit is located
between the first and second oxidation potentials of the por-
phyrin core for Open-In and Open-Ir(CO)Cl and this is not the
case for Open-Zn. Interestingly this difference leads to the fact
that oxidative aromatic coupling occurs much easier for Open-In
and Open-Ir(CO)Cl than for Open-Zn.
this assignment is given by cyclic voltammetry and thin-layer
spectroelectrochemistry (Fig. S1–S10, ESI†). The electrochemi-
cally mediated oxidative couplings of Open-H2 and Open-InCl
have been performed on the 5–10 mg scale and Fused-H2 and
Fused-InCl were isolated in good yields by chromatography.
In conclusion, the use of electrochemical oxidation allows
one to overcome problems with oxidative aromatic coupling of
free-base porphyrins using chemical one-electron oxidants. In
contrast oxidative aromatic coupling can be driven by electro-
chemistry giving rise directly to a fused porphyrin as a dication.
Finally even indium(III) and iridium(III) porphyrins, possessing
relatively high oxidation potentials, were successfully coupled to
the p-extended porphyrins for the first time. The electrochemical
and spectroelectrochemical data seem to demonstrate that
oxidative aromatic coupling does not occur after formation of
a radical-cation but only after formation of a bis-radical where
the first electron is abstracted from the porphyrin p ring system
and the second from the electroactive naphthalene group.
The authors thank the Robert A. Welch Foundation (K.M.K.,
Grant E-680) and the Foundation for Polish Science (TEAM
2009-4/3).
The mechanism for conversion of Open-H2 to Fused-H2 is
similar to that described above for the other Open porphyrins
but it differs from the metallated complexes in that the final
electrochemically generated Fused product is produced as a
mixture of the free-base porphyrin with an extended p ring
system, Fused-H2, and the diprotonated Fused free-base por-
phyrin represented as [Fused-H4]2+ (Scheme 3). Evidence for
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